1. Field of the Invention
This invention relates to overhead projectors, and in particular, those of the transmissive type.
2. Description of the Prior Art
The use of multi-element lenses in a light condensing and illuminating system is well known in small format projection devices such as movie and slide projectors. In these cases the illumination leaving the lenses is convergent and is directed through a film or slide gate. There have been many attempts to increase the uniformity of illumination through this film gate, while at the same time minimizing the spherical aberration of the light source image at the entrance pupil of the projection lens. Especially for high numerical aperture condensing systems, the well-known COS.sup.4 Law of Illumination makes it difficult to achieve uniform illumination.
The relationship of illumination uniformity, spherical aberration, and the COS.sup.4 Law of Illumination is well described in Wallin, W., "Design of Special Projector Illumination Systems", Journal of the SMPTE, October, 1962, pp. 769-771 and Weiss, H., "Wide-Angle Slide Projection", Information Display, September/October, 1964, pp. 8-15.
Several methods have been proposed to increase the illumination uniformity of convergent light in small format condenser systems. Rantsch, U.S. Pat. No. 2,186,123 places a pair of multi-lens arrays between a pair of spherical glass condenser lenses to increase the light uniformity at the film plane. Miller, U.S. Pat. No. 3,051,043, tilts and decenters the light source and pair of condenser lenses relative to the optic axis to improve the uniformity at the film gate. Osterberg et al., U.S. Pat. No. 2,637,242, uses a pair of aspheric surface glass lenses to increase the illumination uniformity at the film gate and preserve the focusing accuracy. Hund, DE Patent 3,505,771, also used aspheric surface glass lenses to produce convergent illumination and improve the illumination uniformity at the film gate.
Overhead projectors have a much larger stage or film gate than considered in the above-described systems. Therefore any practical combination of condenser lenses normally produces divergent illuminating light to fill the stage area. Convergence of the light to the projection lens is then accomplished by the use of a large-area Fresnel lens located adjacent to or forming the stage plane. The purpose of the condenser is to illuminate the stage area, and this is usually accomplished by a single glass lens. This is generally a positive meniscus lens, or a plano-convex lens with a spherical or aspherical surface. One exception is a two-element glass system described by E. Hubner (U.S. Pat. No. 3,486,817) in which convergent illumination forms a secondary image of the light source close to the condenser at the position of a folding mirror.
It is an intent of the current invention to provide a highly uniform light distribution at a large format film gate, such as that for an overhead projector. This uniformity is especially important for the use of LCD (liquid crystal display) projection panels, which are placed on the stage of overhead projectors. The non-uniformity of illumination at the stage produced by many high intensity overhead projectors causes uneven heating of the liquid crystal material across the stage plane. This, in turn, causes uneven contrast in the projected image.
It is also an intent of the current invention to provide a light condensing system that collects radiation from the light source over a wide angle. This is especially important for isotropic radiating light sources, such as a high-intensity plasma discharge lamp.